In electrographic printing and copying, toner compositions are applied to an electrostatic latent image formed on a dielectric surface in order to develop, i.e. make visible, the image. The dielectric surface may be a coating on a sheet or web of paper to which toner is applied. Alternatively, the dielectric surface may be the charge retentive surface of a drum, belt or the like from which toner applied thereto is transferred to a sheet or web of plain paper. The electrostatic latent image may be established through electrostatic induction by a charged writing head, by ion projection, or through photoconduction, as in electrophotographic copiers or laser printers. Typically, the toner composition is a liquid having pigments or dyestuffs combined with a plastic or resinous binder, hereafter called "solid pigment particles" or "colorant", with very small amounts of added charge control agents, and dispersed in a large volume of liquid dispersant, primarily solvent. One commonly used solvent is an isoparaffinic hydrocarbon available under the tradename ISOPAR manufactured by Exxon Corporation. Multi-color electrostatic printers typically store liquid toner in storage tanks, one for each desired color, and selectively dispense the toner to one or more applicators as it is needed, to form a composite color image. Usually, any excess toner is returned to the appropriate supply tank for reuse.
In conventional electrostatic printers and copiers using liquid toner, the toning applicator must be prewet before it is used since it often dries out between use. Further, toning applicators must be periodically cleaned to remove toner particles caked or dried onto the shoe. Often this cleaning is done by hand with large amounts of a cleaning solvent, sometimes as frequently as once a day. This is a considerably messy task and results in a long time during which the printer or copier is not in use.
In U.S. Pat. No. 4,157,219, Ohta et al. describe a multicolor electrostatic imaging apparatus wherein different color liquid developers are supplied to a medium bearing an electrostatic latent image. The apparatus comprises leading and trailing squeeze rollers in horizontally spaced relation, a receptacle having a plurality of valved pipes connected to respective liquid tanks for applying a plurality of liquid developers and rinse liquid to the medium, and a carriage charging device disposed at the rear side of the trailing roller for neutralizing or dissipating the residual charge remaining on the medium after development. These components are mounted to a reciprocating carriage so that the latent image is successively subject to developing, squeezing and charging during carriage movement in a forward direction and to rinsing during carriage movement in the reverse direction. Such movements are repeated some desired number of times to make a multi-colored print.
In U.S. Pat. No. 3,129,115, Clark et al. describe a xerographic developing apparatus having two reservoirs of toner which are typically used separately. The first reservoir supplies developer liquid to a xerographic plate to form an image, while the other reservoir supplies liquid for cleaning the plate. All fluid from the xerographic plate is channeled into a third tank. Electrode plates within the third tank are charged to attract developer particles such that fluid with a predominance of particles is funneled into the first reservoir, while liquid with a significantly lower concentration of particles is funneled into the second reservoir. An object of the present invention is to provide an improved method of washing away residual solid pigment particles from common circulating and dispensing portions of an image developing apparatus of the type using liquid toner.